The invention relates to a separator for sorting particulate material into a fine fraction and a coarse fraction, the separator comprising a rotor with a substantially vertical axis and vanes, an adjustable guide vane system surrounding the rotor and a housing encasing the guide vane system and the rotor and having one or more inlet ducts for the supply of a conveying gas and unsorted material, an outlet duct from at least one end of the separator for discharging the separated fine material fraction suspended in the conveying gas and an outlet duct beneath the rotor and the guide vane system for discharging the separated coarse material fraction for further treatment. Such a separator is hereinafter referred to as of the kind described.
In such a separator the material to be sorted in the separator may either be introduced into the separator separately from the conveying gas into which it is then suspended just inside the guide vane system, or be presuspended in the conveying gas outside the separator so as to be fed together with the gas through the guide vane system.
In both cases the material/gas suspension is passed into the rotating rotor vanes where, as a result of the centrifugal force acting upon the material, the coarser material fraction is flung outwards towards the guide vane system.
The centrifugal force may be increased by adjusting the guide vanes so that they form a smaller angle to the tangential direction of the rotor.
The coarser particles in the suspension flung outwards towards the guide vane system will fall down along the inside of the guide vane system to the bottom of the separator to be discharged therefrom, while the finer particles remaining suspended in the gas stream are conveyed through the rotor and out through the outlet duct from the separator and subsequently from the conveying gas e.g. by a precipitator arrangement.
The separated fine fraction consists of nearly all particles below a certain first, smaller grain size, while the coarse fraction consists of nearly all particles above a certain second, larger grain size. Additionally, in both fractions there will be distribution of an intermediate fraction comprising grain size between the first and second grain sizes.
The distribution of particles of intermediate size is a measure of the inability of a separator of the kind described to provide desired precise grain size cut-off point ("cut size"), such that all particles below a certain size pass through the rotor and all particles above that size are flung towards the guide vane system.
The difference in size between the first and second grain sizes indicates the separation sharpness of the separator in question, and this separation sharpness as well as the cut size of the separator, are characteristics of the design of such a separator.
Control both of the cut-size and the separation sharpness of a separator are desired in such cases where a certain grain size distribution in the finished product (the fine fraction) must be obtained, e.g. when grinding cement, where a quite definite grain size distribution is a precondition to the strength properties and workability of the concrete to be produced from the cement.
The cut-size may be controlled by adjusting partly the rotation speed of the rotor and partly the inclination of the guide vanes, while the control of the grain size distribution in the finished product from a separator plant has hitherto required the use of two or more separators working in parallel or in series and being preset for different cut sizes.